The present invention relates in general to active power filters, and in particular to a new and useful apparatus and method for compensating nonlinear currents drawn by different loads such as electronic power supplies or motor drives which are known to draw nonsinusoidal (nonlinear) currents.
The problem of nonsinusoidal line currents created by nonlinear power electronic loads is well documented. The pulsating currents drawn by uncontrolled rectifier bridges are often corrected using a "high power-factor preregulator", which is an uncontrolled rectifier loaded by a boost converter. High power-factor preregulators are discussed in L. H. Dixon, Jr., "High power factor preregulators for off-line power supplies," Unitrode Power Supply Design Seminar, Topic 6, 1988. The boost converter is controlled to draw a current from the rectifier which has the same shape as the input voltage. The boost converter must process all of the load power.
Sliding-mode control is concerned with forcing one or more variables (often, but not necessarily, state variables) to follow a specific trajectory. See, J. J. Slotine and W. Li, Applied Nonlinear Control, Englewood Cliffs, N.J.: Prentice-Hall, 1991. The trajectory is known as the sliding surface. The location of the variables relative to the sliding surface governs the control law which is applied to the system. As the system variables of interest pass through the sliding surface, the control law changes. The nonlinear control law is chosen so that regardless of where the system is with respect to the sliding surface, control actions always drive the system toward the sliding surface. Power electronic systems are natural candidates for sliding-mode control because the topology of the circuit changes with the operation of the switches. See, A. Sabanovic, N. Sabanovic and O. Music, "Sliding mode control of dc-ac converter," IEEE Power Electronics Specialists Conf. Rec., pp. 560-566, 1988. This "varying structure" of the system leads to the nonlinear control law which forces the system back to the sliding surface. Sliding-mode control of a three-phase active power filter is discussed in P. F. Wojciak and D. A. Torrey, "The design and implementation of active filter systems using variable structure system concepts, "IEEE IAS Annual Meeting Conf. Rec., pp. 850-857, Houston, Tex., 1992; the present invention expands and modifies this control law which was co-authored by the present inventor.